A magnetic disk for a hard disk drive is known as one type of information recording medium. The magnetic disk is a disk having a center hole and is fabricated by superimposing magnetic films on the surface of a glass substrate. The magnetic disk is rotated by a spindle received in the center hole. Information recorded on the magnetic disk is read by a magnetic head, which moves along the surface of the magnetic disk in a state levitated from the surface by a certain distance.
It is desirable for the magnetic disk to have a high recording density to increase the recording capacity of the magnetic disk. Japanese Laid-Open Patent Publication No. 2002-251716 describes a method for increasing the recording density of a magnetic disk by forming ridge-shaped projections, or a texture, extending in the circumferential direction along the surface of a glass substrate so as to increase the magnetic anisotropy. More specifically, material, or a glass plate, for forming a glass substrate is prepared. The surface of the glass plate is polished in a number of stages to be smoothed. Then, a scrubbing tape scrubs the surface of the glass plate in the circumferential direction while supplying the surface with an abrasive to form the texture.
In comparison with a glass substrate that does not have a texture, a textured glass substrate resists the adhering (sticking) of a moving magnetic head to the disk surface. Since the textured glass substrate resists sticking, the distance between the disk surface and the head may be decreased. Generally, decrease in the distance between the disk surface and the magnetic head enables an increase in the recording density of a magnetic disk. Thus, the textured glass substrate is advantageous for increasing the recording density.
However, the formation of a texture having projections with uniform apexes is difficult, and an uneven texture having deficiencies such as abnormally tall projections may be formed. When the distance between the disk surface and the magnetic head is small, with a magnetic disk formed from a glass substrate having an uneven texture, the moving magnetic head may crash against or become caught in the abnormal projections thereby causing glide errors. In such a magnetic disk, the distance between the disk surface and the magnetic head must be increased to avoid the occurrence of glide errors. In other words, the levitation height of the head for such a magnetic disk is high. Accordingly, there is a shortcoming in that even if textured glass substrates are manufactured through the same process, the head levitation height differs depending on whether the texture has a deficiency.
To reduce deficiencies in the texture, the surface of the glass substrate is rubbed with a hard sponge or is etched. By rubbing the surface of the glass substrate with a hard sponge to partially remove abnormal projections, abnormally tall projections are removed. However, there is a possibility of the surface of the glass substrate being scratched. By etching the surface of the glass substrate, the top portions of abnormal projections are isotropically etched, and abnormally tall projections are removed. However, the etching eliminates small (normal) projections and pits. Thus, there is a possibility of the texture being shaped differently from the desired shape. Scratches in the disk surface or a texture having a different shape are factors that cause glide errors or sticking. This increases the crashing frequency of the magnetic head and the frequency the magnetic disk is scratched. Thus, a glass plate having the desired texture cannot be manufactured with a high yield.